Improve custom importer to pass parent import context

One example would be a custom importer that loads the
content from the web. If this includes another import,
it should be loaded relative to the parent file. To do
this, the custom importer needs to know the context of
the parent import. Which is what this commit will fix!
Merge pull request #691 from mgreter/feature/importer-parent-context

Improve custom importer to pass parent import context